The disclosure relates generally to a power generation system and more specifically to systems and methods for maintaining synchronism between a power generator and a power grid in the power generation system.
Distributed energy resource (DER) systems are small power generators, typically in a range from 3 kW to 10,000 kW, that generate power from various sources and transfer the generated power to a power grid connected to the power generators. The power grid collects the power generated from multiple power generators and transmits the power to different locations. Typically, the DER systems are an alternative to or an enhancement of traditional electric power systems. Small power generators may be powered by small gas turbines or may include fuel cells and/or wind powered generators, for example. The DER systems reduce the amount of energy lost in transmitting electricity because the electricity is generated very close to where it is used, perhaps even in the same building. DER systems also reduce the size and number of power lines that must be constructed.
Until recently, network operators in many countries allowed small distributed generators to quickly disconnect from the network in case of severe network disturbances. Network disturbances may be due to several kinds of faults that occur during operation of the DER systems. Typically, the faults in electric power grids may be either balanced faults or unbalanced faults. In practice, most of the faults in power systems are unbalanced single phase faults. When a fault in the utility system occurs, voltage in the system may decrease by a certain amount. Such decreases in the voltage may be referred to as “voltage dips” or “voltage sags.”
The characteristics of such “voltage dips” or “voltage sags” depend on several aspects, such as type and severity of the fault, location of the fault, and duration of the fault. Typically, the magnitude of the “voltage dip” or “voltage sag” at any location in the power grid may depend on the severity of the fault and the distance to the fault. Similarly, the duration of the “voltage dip” or “voltage sag” may depend on the time required for the protective circuits to detect and isolate the fault. The duration of the “voltage dip” or “voltage sag” may be usually of the order of a few hundred milliseconds.
Further, in an event of a fault, the sudden reduction of the voltage at the point of interconnection of the generator and the power grid may result in a sudden reduction of the electrical power output of the generator. As a consequence, the unbalance between the electrical power output of the generator and the mechanical power input from the engine may cause the acceleration of the generator, which may lead to loss of synchronism between the generator and the rest of the grid. Thus, certain types of generators with small inertia may accelerate rapidly and lose synchronism during fault events. In a non-limiting example, certain types of generators include small synchronous or asynchronous generators.
In the past, under these inadvertent fault and large power disturbance circumstances, it has been acceptable and desirable for small generators to trip off line whenever the voltage reduction occurs. Operating in this way has no real detrimental effect on the stability of the power grid when the total power provided to the grid from these small generators is very small compared to the total power provided to the grid by all other power generating units. However, as penetration of small distributed generators in the grid and the amount of power provided to the grid by these small distributed generators increases, the stability of the electric grid may be jeopardized if all such generators are disconnected during a fault event with low voltage conditions. It is therefore desirable for these generators to remain synchronized to the grid, to ride through low voltage conditions, and to be able to feed electric power into the grid immediately after the fault is cleared. Therefore, emerging grid codes are increasingly requiring small generators to “ride through” certain voltage conditions caused by grid fault events. This, however, currently represents a big challenge for generators with small inertia, which tend to rapidly accelerate after a “voltage dip.”
Various techniques may be employed to overcome the issue of rapid acceleration in power generators during fault conditions. One such technique is to provide a mechanical braking to halt a prime mover in the power generator. However, the mechanical braking units have a relatively slow reaction time and are therefore inadequate for small generators with small inertia, which could lose synchronism even before the mechanical braking is applied. Another technique is to increase the inertia of the generator, for example by adding a flywheel, to reduce the generator acceleration during low voltage conditions. This technique results in reducing the dynamic performance of the generator with additional weight and cost. An alternative technique is to provide an electrical braking with a braking resistor to dissipate power in the resistor in order to halt the acceleration of the power generator during fault conditions. However, most of the electrical brake techniques include expensive power electronics that substantially increases the cost of the system.
Hence, there is a need for an improved system and method for an effective and inexpensive Fault Ride Through (FRT) power generation system to address one or more aforementioned issues.